The Energy/Water Nexus

Sustainable energy can fall in two buckets. One comprises all the means to lower the carbon footprint of current energy sources. This would include clean coal, using natural gas in place of coal to produce electricity, combined cycle approaches to energy production, and the like. The second bucket is that of renewable energy. The outstanding examples of that are biofuels, wind energy and solar energy.

Each of the foregoing has very different water utilization. One billion persons do not have access to drinking water. Should efficiency of water utilization be a factor in our choice of alternatives, and not just carbon footprint? Going further, should water usage be a litmus test in areas in which the citizenry suffer a high level of privation? This was the subject of the RTEC Breakfast Forum on June 15, 2010.

We tend to use fresh water for everything when something less could do the job. This is likely an artifact of water being relatively cheap. If some of the major users were able to tolerate less than fresh water, water would be freed up for human consumption. An extremely topical area for this thought is shale gas drilling in the US. Each well uses up to 5 million gallons per well as the main component of fracturing fluid. Only about a third of the fluid used returns to the surface. Currently it cannot be re-used because of contaminants, salt in particular. Even if this were to be cleaned up for re-use, the other two thirds would need to be made up from fresh water sources.

Fortunately, industry is taking a hard look at the problem and is moving to modify formulations to be able to tolerate significant salinity. So, not only would the flow-back water be re-usable, but other saline waters of convenience, such as sea water, come into play. In an odd twist, it turns out that salinity is actually good for the operation (it stabilizes the clays). Lemonade from lemons, as it were.

While not particularly applicable to the shale gas play in the eastern United States, a lot of “tight gas” exploitation occurs in the middle of the country, in areas that are severely drought prone. Here, water for energy competes with that for agriculture. The ability to tolerate salinity would be huge. This is because saline aquifers are plentiful. Supporting technology would be required in areas such as benign biocides. Bacteria in these waters are often pernicious, some being sulfate reducing, and thus producing hydrogen sulfide in situ when used for fracturing fluid. But these are all tractable if the major issue of some level of salinity is traversed and if innovations in cost effective water treatment are forthcoming.

The key to water treatment is to have a fit-for-purpose output. Potable water is the most expensive. An intermediate product could be adequate and meet the economic hurdles. Today almost all desalination approaches have fresh water as the output.

Agriculture tolerant of brackish water is a new area without significant currency today. The most obvious example is algae for bio fuel production. Algae, of course, thrive on salt water (and consume carbon dioxide as another plus). A class of plants known as halophytes make themselves saltier than the salt water, thus causing fresh water to flow into them by osmosis. Most such would likely be for biomass for energy production, not food.

Water used in conventional energy production is also highly variable. The paper by Mulder et al describes water efficiency of different energy production methods. Any eye-opener is the significant difference between closed and open loop cycles. An interesting nuance is also the difference between water withdrawal and water use. For example, if a facility such as a nuclear plant, withdraws water from a river, and then returns hotter water, the subsequent evaporation downstream is not counted in some measures. The withdrawal number remains low, even though the net usage was higher.

Using less water is not always productive. Apparently in some areas drip irrigation leads to salt build up around the plant. Also, drip irrigation returns no water to the aquifer. But on balance that must still be more effective than spraying, where evaporative losses may not necessarily be returned as convective rainfall.

Drought tolerant biomass is highly touted these days. Jatropha in India and elsewhere is seen as an important crop for biodiesel production. However, an interesting twist on this is that these plants can tolerate drought, but they grow much faster with more water. A farmer with water access will draw on it. So, what is needed is clever business models and associated policy drivers to encourage water conservation in the face of a compelling economic driver to use more. An interesting problem for a behavioral economist.